Various types of these direct current boost converters are widely known, and they are divided into those using one coil and those using two coils.
FIG. 1 shows an example of a conventional direct current boost converter using one coil. The coil (1) and the first transistor (2) are connected in series between the positive terminal and negative terminal (grounding terminal) of a DC power supply, and the node of this coil and the first transistor is connected through the diode (3) to the node of the EL panel (4) and the second transistor (5). The other terminal of this EL panel (4) and the second transistor (5) is connected to the negative terminal of the DC power supply. The clock signal shown in FIG. 2A is applied to the gate of the first transistor (2), and a gate signal with a repeating frequency which is lower than the clock signal shown in FIG. 2B is applied to the gate of the second transistor (5).
With respect to this example of a direct current boost converter, when a clock signal is applied to the gate of the first transistor (2) during the period in which the second transistor (5) is off, a voltage which gradually increases, as shown in FIG. 2C, is applied to the EL panel (4).
FIG. 3 shows another example of a conventional direct current boost converter using one coil, and this converter is described in the detailed explanations of U.S. Pat. Nos. 4,527,096 and 4,208,869. The coil (1) and first transistor (2) are connected in series between the positive terminal and negative terminal (grounding terminal) of the DC power supply, and that node is grounded through the diode (3) and the capacitor (6). The cathode of this diode (3) and a first plate of the capacitor (6) are connected to the switching bridge circuit which is composed of the second and third transistors (5a) and (5b), connected in series, and the fourth and fifth transistors (5c) and (5d), connected in series, and is connected between the node (A) of the EL panel (4) and the second and third transistors (5a) and (5b), and the node (B) of the fourth and fifth transistors (5c) and (5d).
The clock signal shown in FIG. 4A is applied to the gate of the first transistor (2), and the gate signal shown in FIG. 4B is applied to the gates of the second and fifth transistors (5a) and (5d), and the second gate signal having the opposite phase to the first gate signal, as shown in FIG. 4C, is applied to the gates of the third and fourth transistors (5b) and (5c). Therefore, the second and fifth transistors (5a) and (5d) go on simultaneously, and then the third and fourth transistors (5b) and (5c) go on simultaneously. As a result, a boosted voltage is applied to the nodes (A) and (B) as shown in FIGS. 4D and E.
FIG. 5 shows another example of a conventional direct current boost converter using one coil, and this is described in the detailed explanation of U.S. Pat. No. 5,313,141. The coil (1) and series circuit of the first and second switching elements (2a) and (2b) is connected between the positive and negative terminals of the DC power supply, and the node between the first switching element (2a) and the coil (1) is connected to the cathode of the first diode (3a), while its anode is connected to the third switching element (5a), the node of the second switching element (2b) and the coil (1) is connected to the anode of the second diode (3b), while its cathode is connected to the fourth switching element (5b), these third and fourth switching elements are connected to one terminal of the EL panel (4), and the other terminal of the EL panel is grounded.
A clock signal as shown in FIG. 6A is applied to the first and second switching elements (2a) and (2b), and the first and second gate signals which have the opposite phase to one another, as shown in FIG. 6B and C, are applied to the third and fourth switching elements (5a) and (5b). As a result, a boosted voltage is applied between the two terminals of the EL panel (4) as shown in FIG. 6D.
FIG. 7 shows an example of a conventional direct current boost converter using two coils, and this is described in the detailed explanation of U.S. Pat. No. 5,349,269. The series circuit of the first coil (1a) and the first transistor (2a) is connected between the positive and negative terminals of a DC power supply, and the node of this coil and transistor is connected to one terminal of the EL panel (4) through the first diode (3a). The node between the first diode (3a) and one terminal of the EL panel (4) is grounded through the second transistor (5A). Also, the series circuit between the second coil (1b) and the third transistor (2b) is connected between the positive and negative terminals of a DC power supply, and the node between this coil and transistor is connected to the other terminal of the EL panel (4) through the second diode (3b). The node between the second diode (3b) and the other terminal of the EL panel (4) is grounded through the fourth transistor (5b).
The clock signal shown in FIG. 8A is applied to the gates of the first and third transistors (2a) and (2b), and the first and second gate signals of opposite phase to one another are applied to the gates of the second and fourth transistors (5a) and (5b). As a result, a boosted voltage is applied between the two terminals of the EL panel (4) as shown in FIGS. 8D and 8E.
With respect to the conventional direct current boost converter shown in FIG. 1, although the structure is simple because it only requires a single coil, its weakness is that the voltage which is applied to the EL panel (4) is of single polarity, and the efficiency of light-emission is low.
With the conventional direct current boost converter shown in FIG. 3, one coil is sufficient as well, and although it has the advantage that in addition to the structure being simple, a voltage of alternating polarity is applied to the EL panel (4), making the efficiency of light emission high, because the capacitor (6) is charged with a driving voltage, when the driving voltage becomes high, it requires a capacitor with a high resistance to voltage, and its weakness is its size and cost.
With the conventional direct current boost converter shown in FIG. 5, one coil is sufficient as well, and since a voltage of dual polarity is applied to the EL panel (4) it has the advantage that the efficiency of light emission is high, but when the transistors which compose the switching elements (5a) and (5b) are composed of integrated circuits, a negative voltage is applied to a semiconductor substrate, so there is the need to construct it to prevent the flow of electricity into the semiconductor substrate, which makes the structure of the integrated circuit complex, resulting in the weakness of high cost.
With the conventional direct current boost converter shown in FIG. 7, because the voltage which is applied to the EL panel (4) alternates in polarity, the efficiency of light emission is high; however, since it requires two coils (1a) and (1b), it has the weakness of complex structure and high cost.
The type of drive circuit described above is well known from, for example, the detailed explanation of U.S. Pat. No. 5,349,269, and its structure is shown in FIG. 9. As shown in FIG. 9, the series circuit of the first coil (1a) and the first transistor (2a) is connected between the positive and negative terminals of the DC power supply, and the connection point between this coil and transistor is joined to one terminal of the EL panel (4) by way of the first diode (3a) and the first Zener diode (7a). The connection point between the first Zener diode (7a) and one terminal of the EL panel (4) is grounded through the second transistor (5a). Also, the series circuit of the second coil (1b) and the third transistor (2b) is connected between the positive and negative terminals of the DC power supply, and the connection point between this coil and transistor is joined to the other terminal of the EL panel (4) through the second diode (3b) and the second Zener diode (7b). The connection point between the second Zener diode (7b) and the other terminal of the EL panel (4) is grounded through the fourth transistor (5b).
A clock signal is applied to the gate of the first and third transistors (2a) and (2b), and gate signals that have opposite phases from one another are applied to the gates of the second and fourth transistors (5a) and (5b). As a result, a boosted voltage is applied between the terminals one and two of the EL panel (4).
With the widely known drive circuit described above, because the first and second Zener diodes (7a) and (7b) are connected to the current path between the first coil (1a) and the EL panel (4) and the current path between the second coil (1b) and the EL panel (4), these Zener diodes (7a) and (7b) serve the purpose of preventing electrical leaks from the DC power supply. Consequently, when the second transistor (5a) is in a conductive state, the path through the positive terminal of the DC power supply, the first coil (1a), the first diode (3a), the second transistor (5a), and the negative terminal is broken by the Zener diode (7a). When the fourth transistor (5b) is in a conductive state, the path through the positive terminal of the DC power supply, the second coil (1b), the first diode (3b), the fourth transistor (5b), and the negative terminal is broken by the Zener diode (7b). As a result, through the action of these Zener diodes to prevent electrical leaks, the effect of greatly reducing the loss of energy is achieved.
However, when the above drive circuit is built into an IC substrate, a parasitic transistor is formed in the structure of the Zener diode between it and said IC substrate, and a high-speed, in other words a high-frequency, spike flows into the substrate side through this parasitic transistor, and eventually flows to a ground, with a resulting a loss of energy. When this type of energy loss occurs, the benefit of placing a Zener diode cannot be realized.